>What would people recommend for the fastest way to get a prototype compiler>working? GCC or LCC? Another compiler with a portable backend?

Just for microcontrollers I'd suggest Forth, if this satisfies your
assumptions on a "compiler".

To make Forth work on a new target machine, it's sufficient to
translate about 20 words into machine code, then a Forth system can be
implemented in Forth itself. Using Forth, you can implement an
assembler for the target machine, and use existing Forth libraries to
translate source code from e.g. infix to UPN, or to accept subsets of
C, Fortran, Pascal or other more common languages.

I implemented and used Forth myself, on a broad range of 8 and 16 bit
(micro)processors, up to PDP11's. Just on small systems, like embedded
controllers, Forth can be used to develop and debug applications on
the target systems, no need for ICE's or other hardware. Execution
speed of the compiled programs is almost acceptable, since every other
compiler also would use subroutines for extended arithmetic, on 8 bit
hardware.